1. Field of the Invention
The present invention relates to a self-tack wrapping film comprising a chlorine-free resin. More particularly, it relates to a self-tack wrapping film comprising chlorine-free resin and having improved tackiness, heat resistance, microwave oven suitability and cutting characteristics, and appropriate flexibility.
A self-tack wrapping film is capable of preventing foods from evaporation of water during storage in a refrigerator, evaporation of water during a heating in a microwave oven, diffusion of flavor or taste, or getting scented with other odors during storage, or getting dusty, and therefore largely used in families, restaurants and hotels for hermetically sealing foods.
With recent wide-spreading use of a microwave oven, demand in this use has remarkably increased. That is, a self-tack wrapping film is largely used for hermetically sealing foods placed on a ceramic- or glass-made container to prevent foods from evaporation of water and loss of taste in a microwave oven.
The wrapping film used in this use is required to be excellent in not only adhesion to the container but also tackiness property between films. Moreover, the wrapping film is required to be small in thermal shrinkage and free from perforations, thermal fusion and whitening in a microwave oven. In this specification, these properties are referred to as a microwave oven suitability.
2. Description of the Related Art
As a conventional self-tack wrapping film, those prepared using polyethylene or chlorine-containing resins such as polyvinyl chloride and polyvinylidene chloride as a main component are known.
However, the film made of polyethylene has a defect such that perforations are readily formed when the film is brought into contact with hot oily or fatty materials such as edible meats or fried foods so as to be allowed to stand at a high temperature. The film made of polyvinyl chloride has a problem such that it readily exhibits whitening when brought into contact with boiling hot water. The film made of polyvinylidene chloride is large in thermal shrinkage and more expensive than the above two types of films.
Therefore, none of these conventional wrapping films can be said to be satisfactory in a microwave oven suitability.
The wrapping film is usually used in the form of a film of 20 to 45 cm in width and 8 to 15 .mu.m in thickness, which is rolled on a core material usually made of paper and kept in a case such as a paper box. In the use of the film, the film is drawn out from the case to a desired length, brought into contact with a blade called "saw blade" provided with the case at a portion to be cut out and then pulled to be cut into a suitable length.
As the "saw blade", a simple blade obtained by punching out an iron plate or cardboard having a thickness of about 0.2 mm into a shape of saw is usually used. Moreover, the case supporting the "saw blade" is usually a coated cardboard box having a basis weight of about 350 to 700 g/m.sup.2, which is therefore extremely low in stiffness.
In view of the foregoing, the wrapping film is also required to be easily cut out even by such a simple cutting mechanism. Namely, the film is required to have a good cutting characteristics.
In the case of the film made of polyvinylidene chloride, once a tear appears at a certain position of the film at the time of cutting, the tear extends so as to cut the film not along the "saw blade" but slantingly. In the case of the film made of polyvinyl chloride or polyethylene, elongation of the film at the time of cutting is too large to obtain sharp cutting.
In a meanwhile, the film made of the chlorine-containing resin such as polyvinylidene chloride or polyvinyl chloride is appropriate in flexibility and therefore successfully used in this respect. However, the film contains a large quantity of chloride and therefore produces chlorine gas upon incineration Moreover, the film contains a large quantity of harmful plasticizers. For these reasons, the film cannot be said to be satisfactory also from an environmental point of view.
Thus, a self-tack wrapping film made of a chlorine-free resin has been attempted to be developed. For example, Japanese Patent Laid-Open No. (JP-A-) 6-238848 proposes a film obtained by laminating a terpolymer film layer of ethylene, propylene and .alpha.-olefin having 4 to 8 carbon atoms on one side or both sides of a polycarbonate resin film layer. Further, Japanese Patent Laid-Open No. (JP-A) 6-122182 proposes a wrapping film obtained by disposing a polyolefin resin layer on both sides of a polyamide resin layer, wherein an adhesion layer of a modified resin composition obtained by melt-mixing an acid-modified polyolefin and a polyamide resin is placed between the polyamide resin layer and the polyolefin resin layer.
However, there has never been obtained a self-tack wrapping film of a chlorine-free resin capable of simultaneously satisfying tackiness, heat resistance, microwave oven suitability, "saw blade" cutting characteristics and appropriate flexibility.